We have employed Chinese hamster ovary (CHO) cells containing a human chromosome 11 (termed AL hybrid cells) as a highly sensitive detection system for mutagenesis in an effort to evaluate possible direct mutagenic effects of cigarette smoke condensate (CSC) and its fractions. Cytotoxicity induced by Nmethyl-N'-nitro-N-Nitrosoguanidine (MNNG) in AL cells increased with time of incubation. The 50% inhibitory concentration of MNNG after 1, 3, 6 and 20 hr of incubation was 0.8, 0.4, 0.2 and 0.1 micromoles, respectively. Mutagenicity increased with dose and time reaching a maximum of 1,250 mutants/100,000 survivors (800% above background) after 3 hr of incubation with 2 micromolar MNNG and a maximum of 1,700 mutants/100,000 survivors (1,100 % above background) after 20 hr of incubation with 0.2 micromolar MNNG. The cytotoxicity of CSC increased with increasing incubation time with 50% inhibitory concentrations of 100, 80, 50 and 30 micrograms/ml after 1, 3, 6 and 20 hr of incubation, respectively. CSC mutagenicity increased with time of incubation up to 3 hr with a maximum of 300 mutants/100,000 survivors (250% above control) after incubation with 100 micrograms/ml CSC (p value less than 0.0005, Student's t-test). Cytotoxicity and mutagenicity of CSC were inversely proportional to cell density, while cytotoxicity and mutagenicity of MNNG were unaffected by cell density. The 3 hr incubation time and 50% inhibitory concentration of the acidic fraction of CSC (30 micrograms/ml) induced 350 mutants/100,000 survivors (a 230% increase above background, p value less than 0.0005). The basic and neutral fractions caused a much lower increase at the 50% inhibitory concentration (80 and 200 micrograms/ml, respectively). The possible role of oxy-radicals generated by tobacco smoke condensate and its fractions in mutagenesis of AL hybrid cells is being investigated.